Solid oxide fuel cells (SOFC) are a class of fuel cell characterized by the use of a solid oxide material as an electrolyte, which conducts negative oxygen ions from a cathode to an anode. At the anode, the negative oxygen ions combine electrochemically with hydrogen and/or carbon monoxide to form water and/or carbon dioxide, respectively. Solid oxide fuel cells have a wide variety of civilian and military applications from use as power units in vehicles to distributed and central stationary power generation with outputs ranging from 100 W to 100's of MW, at an energy efficiency ranging from 40 to 60 percent depending on application. Like other types of fuel cells, solid oxide fuel cells can have multiple geometries. A typical planar fuel cell design has sandwich-type geometry, where a dense electrolyte is sandwiched between a porous cathode and an anode. This sandwich type geometry facilitates the stacking of hundreds of cells in series, with each cell typically having a thickness on the order of a few millimeters. Because the ceramics used in SOFC's do not become electrically and ionically active until they reach high temperatures, stacks of cells must typically run at temperatures ranging from about 500° C. to about 1000° C. depending on materials used.
Operation of a fuel cell begins by a reduction of oxygen into oxygen ions at the cathode, followed by diffusion through the solid oxide electrolyte to the anode, where they electrochemically combine with a fuel such as a syn gas (H2 and CO) and/or light hydrocarbon fuel. Electrons are released at the anode and flow through an external circuit back to the cathode, performing electrical work. The anode is typically a porous material having relatively high electronic conductivity. The electrolyte is typically a dense layer of cermet with high ionic conductivity and essentially no electronic conductivity. The relatively high operating temperature of solid oxide fuel cells supports oxygen ion transport through the electrolyte.
The cathode of a fuel cell is typically a porous layer attached to the electrolyte where oxygen reduction takes place, and the ability to generate electricity in fuel cells at high current rates and efficiencies is generally limited by the cathode in a well designed and manufactured cell. Cathode materials must be, at a minimum, electronically conductive, and preferable cathode materials additionally possess at least some degree of ionic conductivity, in order to extend the active area for oxygen reduction beyond the triple-phase boundary (TPB) where electrolyte, oxidant and cathode meet. Identifying the best materials is the subject of significant current effort. Materials such as La1-xSrxMnO3-d (LSM), La1-xSrxFeO3-d (LSF), La1-xSrxCoO3-d (LSC), and La1-xSrxCo1-yFeyO3-d (LSCF) have been investigated in detail. However, because oxygen reduction at the surface of the cathode is a slow reaction relative to hydrogen oxidation, efforts to improve the electrocatalytic nature of cathode materials in order to enhance the kinetics of the otherwise sluggish oxygen reduction reaction (ORR) are also being investigated. The cathode oxygen electrocatalyst has been one of the major limiting factors for energy conversion efficiency, cost, and stability of these devices.
One means of improving fuel cell cathode performance is through the use of a mixed ionic-electronic conductor (MIEC) with a thin coating of an electrocatalytic material residing on the MIEC. Such an approach is disclosed in U.S. patent application Ser. No. 12/837,757 having common inventors with the present disclosure, and which is here incorporated by reference in its entirely. Such a coating of electrocatalytic material as disclosed herein significantly improves overall cathode performance by establishing a dielectric/insulator surface between the MIEC and the electrocatalytic material, which significantly increases electron emissions and greatly accelerates oxygen reduction reactions at the cathode. This promotes increased current rates and efficiencies as the larger volume of available oxygen ions increases the diffusion of oxygen ions through the MIEC and electrolyte, and subsequently generates increased reactions with fuel at the anode/electrolyte interface. The oxygen reduction reaction is in virtually all cases the limiting kinetic reaction.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.